wolfsinc

I have seen a lot of debate, here and on other sites, in regards to which fuel pump to use, PSI vs FLOW and if too much of either can cause problems, so I did what I do best…..I played Geek.

Here is what I have found which has helped me in make my decision to purchase the aftermarket replacement fuel pump for a 1985 Corvette for my 1984 Corvette.

Hopefully this will help others make their decision on which fuel pump to use, not only for the C4 series, but for any vehicle, since the logic appears to be the same regardless of year, make or model of vehicle.

Things to know before you begin down this path:
1) Estimated HP of your vehicle.2) Estimated Weight of the fuel you are runningSo to begin, in my case, I looked up the OEM part numbers and specifications for the 1984 and 1985 Corvette fuel pumps. Referring to the “2000 GM Service parts Operations Manual for 1984 – 1996 Chevy Corvette”

1984 Corvette – 25168719 / EP386
http://au.alibaba.com/product/493560...719-EP386.html
FLOW: 75L/H (75 * 0.264 = 19.8 GPH)
PRESSURE: 0.8BAR (.8 * 14.5038 = 11.60304 psi)
VOLTAGE:: 12V

1985 Corvette – 25116163 / EP241
http://au.alibaba.com/product/424679...16163-For.html
Flow:: 120L/H (120 * 0.264 = 31.7 GPH)
PRESSURE:: 3BAR (3*14.5038 = 43.5114 psi)
VOLTAGE:: 12V

So first when dealing with the fuel pump we should all agree that PSI is a measure of the Fuel Pressure Regulator, not necessarily the Fuel Pump. Yes the fuel pump must be able to supply enough fuel to the Fuel Regulator to maintain the minimum fuel pressure, or the engine will suffer from low PSI and the related issues.

However having too high of a PSI rating on the fuel pump does not mean that the fuel system will get more PSI than it can handle and will effect the fuel rail. The job of the fuel pressure regulator is to do exactly that, regulate the fuel pressure down as needed as to ensure the injectors/carburetor can maintain the optimal PSI needed. So the only risk is that a severely high PSI might strain or damage the Fuel Pressure Regulator and thus cause other problems in the injectors and possible more. Again this would have to be a severely high PSI, I would hope anyway. So a fuel pumps PSI rating is a guideline as to what pressure range the pumps flow will obtain under optimal conditions.

So PSI is built by flow through the Fuel Pump, thus the more Flow the more pressure which can be generated, and the more fuel the engine receives, or gets redirected back to the gas tank by the fuel pressure regulator.

Now after reading several sites I kept coming back to the information on Custom-Car.us and Magnum Force because they were easy to understand and confirmed what other sites said in a more sophisticated manner.

Essentially yes having too much flow can cause problems as well as too little flow, especially with performance and fuel efficiency.

How can too much flow cause problems?

1) The Fuels Octane levels are diminished when introduced to heat, which can slightly diminish the octane level in the fuel, resulting in less performance, fuel efficiency and under extreme conditions can cause rough idle or sluggish performance, especially when older gas is not fully utilized before refilling with fresh gas. i.e. filling up continually when the tank is 3 / 4 full. This can leave the diminished gas in the tank making it appear as though other problems exist. If you run the tank below 1 / 8 of a tank, than fill up and notice an initial increase in performance, which degrades quickly over the first 1 / 8 to 1 / 4 tank, then you may be running a fuel pump which has too much flow.2) Fuel moving higher flow rates can cause more foam build up in fuel reservoirs, especially in lower psi systems such as the crossfire, resulting in more breakdown of the fuel and additional air being introduced into the system reducing the desired stead and constant flow of the fuel.

So for general operations you want to try and keep the fuel flow as close to the recommended GPH rating as possible, however for performance use you would want to have the fuel flow at no more than 33% higher than then recommended fuel flow rate.

So how do we calculate the recommended fuel flow rate?

Using the rated HP of Crossfire at 205HP running on standard unleaded fuel we would use the formula:
To calculate the maximum recommended GPH for performance conditions we would use the following formula:
So using these calculations the first thing we can see is that the 1984 Vette EP386 Fuel Pump falls within these limits, however it also shows that it falls under the max recommended flow for performance. In other words at higher RPM’s the fuel pump may struggle to supply enough fuel to the fuel rail.

While the 1985 Vette EP241 is rated higher than the maximum it is not excessively over rated, coming in at approximately 11% higher than the recommended maximum.

So while it might have the potential to degrade the octane level, or add foam, it is not likely that it would since it is not a severely over rated fuel pump. To keep this in perspective the additional 11% overage means and increase of 55 HP will result in a decrease of almost 11% making the maximum recommended rating perfect for the EP241. Simple mods such as Performance EPROM, Air intake systems and performance exhausts can give the additional 55 HP with little cost or effort. In my case I am staring with a K&N air filter, racing plugs and the EPROM so I will have an increase of about 35HP with an investment of about $210.00.

I was not able to confirm whether the 1984 Vette’s OEM fuel pump was a kinetic (centrifugal) pump or not. While I noted that several of the EP386 replacement pumps were in fact positive displacement pumps. The positive displacement pumps will definitely help provide a more steady flow resulting in a more stable idle and reduced cold starts. So if in doubt and afraid to make the jump to a 1985 Vette Fuel pump, change out your OEM pump with replacement pump, which clearly indicates the pump is a positive displacement pump. If the original OEM pump was a kinetic pump you should notice the difference in idle and reduced cold starts.

For those who have lots of mods and are obtaining much more HP, I strongly recommend doing in depth research on running 2 fuel pumps, if you haven't already done so. At first glance I would probably use one of the following configurations, depending on the Fuel Pressure requirements.

For a 650 HP engine:
These would appear to be a nice setup to allow for normal operation as well as performance. Again I am no expert so I would do further research before investing my time or money into either of these setups. But I think this makes for a good starting point and perhaps will help me in asking the right questions, if/when I ever do reach 650HP.

Well I hope this helps someone besides me.

Again sorry for being so long winded, but I had to do the work anyway, might as well share my findings. I am not an fuel pump expert, although I referenced several sites and resources on the topic to obtain these reasonable conclusions. I hope they are fairly accurate.

jhammons01

Damn dude......

too much time on your hands, but thanks for the input.

84VetteMan205hp

sooo is the 85 fuel pump better for an 84 or...

wolfsinc

Ran into other financial problems, namely a costly divorce. I bought the 85 replacement pump, went to install it and ran into more problems with the electrical system surrounding the fuel system. Basically the ECM or ECU is not kicking the fuel pump relay on. I tried wiring the pump directly to 12V, but it appears that something is shorted out either in the ECM or the wiring from the relay to the ECU.

My goal is to get it running within the next month or so, since most of the divorce issues are starting to iron themselves out. But as my luck would have it, I finally buy my Vette and it is sitting in the garage with the hood up, not able to run..... But it will get there.

84VetteMan205hp

good luck